Since the advent of electronics, advances in fabrication techniques have driven the development of faster, smaller and more efficient devices. However, most of these advances have been focused on rigid, wafer-based electronics. Recent interest in wearable electronics, human/machine interfaces, and soft robotics, among other areas, has fueled the development of an entirely new class of electronic devices—stretchable electronics. One device of particular interest is strain sensors that are both highly conformal and highly extensible, i.e., strain sensors that can envelop arbitrarily shaped objects and maintain functionality over large strains. A device having both characteristics could be used to monitor the motion and deformation of irregular and deformable substrates, such as biological appendages, soft actuators and soft robots.
Current commercially available strain gauges, which include a metal strip on a flexible polymer substrate that may be attached to a test specimen, are neither highly conformal nor highly extensible. If a tensile stress is applied to the test specimen, the length of the metal strip increases and the cross-sectional area decreases, which results in an increase in the electrical resistance (R) of the metal strip. The measured change in resistance ΔR may be related to the mechanical strain ε=ΔL/L (where L is the starting length of the metal strip) by the gauge factor GF, which is defined as (ΔR/R)/ε. While widely used, such devices are limited to maximum extensibilities of 10-20%.
Due at least in part to the disparate mechanical properties of soft objects and conventional rigid conductors, combining electronic capability with high degrees of conformality and extensibility (e.g., greater than 100%) has proven to be a difficult problem in terms of both materials development and device fabrication. To date, efforts at producing soft sensors that combine deformable electronics with a stretchable carrier have yielded devices hindered by limited extensibility, high cost, poor durability, low repeatability, inefficient fabrication scalability, and/or limited geometric complexity due to lack of form factor control.